Is your thermostat not working properly? A guide to diagnostics and choosing a new model

The thermostat is the brain of your entire heating system, and if it is not working properly, your bills will rise. The cause of the problem may be unsuitable placement, ageing sensors, oxidised contacts or signal interference. We recommend checking your thermostat as soon as the heating season ends so that any faults can be dealt with in good time.

In our article, you will find a detailed guide to thermostat diagnostics as well as solutions to possible faults.

7 warning signs that your room thermostat is not working properly

The purpose of a thermostat is to ensure the required thermal comfort with optimum energy efficiency. If the temperature does not match the setting or your energy bills are unusually high, you should take notice.

Which specific signs of thermostat failure should you look out for?

1. The thermostat is switching incorrectly (boiler cycling) – frequent switching on and off of the boiler or heat pump is an undesirable phenomenon that leads to excessive boiler wear and higher energy consumption. The problem may lie in the thermostat settings, more specifically in hysteresis being set too low. It should be within the range of 0.5–1 °C. If the hysteresis is lower, the thermostat reacts unnecessarily even to slight temperature fluctuations.
2. Large temperature swings – excessively high hysteresis, i.e. above 1.5 °C, is also problematic. The thermostat reacts to changes with a delay, resulting in noticeable temperature fluctuations and alternating periods of cold and overheating. Such a setting is highly uncomfortable and uneconomical. The greatest savings are achieved by maintaining a stable temperature.
3. The thermostat shows the wrong temperature – if the perceived temperature in the room is significantly lower or higher than the thermostat indicates, the cause may be poor placement. The thermostat has an integrated temperature sensor, and the data from this sensor is used to determine whether the boiler should reduce or increase its output. Draughts or heat sources (e.g. a stove, direct sunlight, appliances) distort the measured temperature and prevent effective control.
4. Delayed response – with underfloor heating, it is normal for the system to respond only hours after a setting change, as it has high thermal inertia. In the case of a heat pump, a delayed response may also be related to a lower heating water temperature. However, with radiators and standard boilers, this is a problem. Check the thermostat settings, especially the temperature hysteresis (ideally 0.5–1 °C), time programmes and predictive smart functions. These settings may be preventing a faster response.
5. Display or Wi-Fi dropouts – the most common cause is missing firmware updates or a weak battery, or possibly incorrect thermostat wiring or a loose cable. However, dropouts may also indicate more serious faults such as oxidised contacts, wear or mechanical damage. If only the thermostat’s Wi-Fi communication is not working, check the signal strength using the test button according to the manual.
6. Frequent or loud relay clicking – the relay makes clicking sounds when switching the heating on and off. If these are too frequent, check the temperature hysteresis setting and make sure the thermostat is installed in a place with a stable temperature. Poor contacts in the digital thermostat terminal block may also be the issue. If the clicking is loud, the relay may be worn or faulty.
7. Disproportionately high bills – high consumption even during a mild winter is usually caused by incorrect thermostat settings or faulty temperature sensors. Check the settings of time programmes and zones, including night setback and setback during absence. Avoid large temperature swings so that the heating system does not have to heat up a cooled house. Also check that there is no source of cold near the temperature sensor (draught, window, door, cold wall) and verify its function by measuring the temperature at the sensor location with a thermometer.

Honeywell DT wired digital thermostat

What hidden causes can there be behind problems with digital thermostats?

The boiler and heating system have been serviced, the room thermostat reports no errors, it is set correctly, yet it still does not work efficiently? There are four possible causes of problems that non-professionals may not identify straight away:

1. Thermostat placement – temperature measurement can be affected by draughts, direct sunlight, certain electrical appliances, a stove or fireplace, or a “thermal shadow” behind a curtain or piece of furniture. Ideally, the thermostat should be positioned so that it is not affected by any sources of heat or cold.
2. Signal interference – wireless thermostats communicate with the boiler via Wi-Fi or a radio protocol. The signal may be disrupted by thick walls and ceilings, metal objects near the thermostat or the receiver by the boiler, as well as other wireless electronics.
3. Oxidised contacts – a common issue especially with older wired models. Contacts oxidise due to moisture, dirt and wear of protective layers. As the contacts degrade, the thermostat may respond with a delay, unreliably or not at all.
4. Sensor ageing – temperature sensors lose measurement accuracy over time. These deviations result in inaccurate temperature control. Sensors must be replaced at the end of their service life, or the thermostat must be recalibrated.

How can you test a thermostat yourself?

Before taking your digital thermostat to a service centre, you can try four simple diagnostic checks yourself. Our clear step-by-step thermostat inspection guide will help you with this.

The “external thermometer” method

This is the simplest way to verify the function of the temperature sensor and whether the thermostat is suitably positioned. For this test, you will need an accurate digital thermometer. Place it as close as possible to the thermostat for at least 24 hours and record the temperatures measured by both the thermometer and the thermostat at regular intervals. Then evaluate the readings:

• The difference between the thermostat and thermometer readings is greater than 0.5 °C → the temperature sensor is probably faulty or worn
• The difference between the thermostat and thermometer readings is less than 0.5 °C, but the temperature fluctuates or is too low or too high → the thermostat is poorly positioned near a source of heat or cold

Boiler switching test

This quick test will help you identify whether the problem lies in the boiler, the relay or the thermostat. On the control unit, increase the set temperature suddenly by 5 °C and observe the reaction of the boiler and relay.

• The relay does not click at all → faulty relay or wiring
• The boiler does not respond to the increased temperature → communication fault between the thermostat and boiler or a boiler fault
• The boiler responds correctly to the increased temperature → look for the cause of the problem in the temperature sensor or thermostat settings

For skilled users only: Checking the thermostat terminal block

Checking the terminal block can help identify incorrect thermostat wiring, loose contacts, oxidation and contamination. These faults cause unstable communication between the thermostat and the boiler. As this involves working with electrical wiring, we do not recommend that it be carried out by non-professionals.

First switch off the power supply to both the boiler and the thermostat and remove the thermostat cover. Visually inspect the terminals and conductors and look for signs of burning, corrosion, dirt or moisture; if necessary, use a multimeter to check the voltage.

If necessary, replace the conductors. Check whether the cables are moving and tighten any loose terminals.

Factory reset

Resetting the thermostat may be the last resort before replacing it. Consider this step if the thermostat freezes, behaves completely illogically or changes settings by itself. After the reset, programme the thermostat again and monitor its behaviour. If the problems persist, consider replacing it.

Siemens Connected Home smart thermostat

When is thermostat repair no longer worth it?

The service life of room thermostats varies by manufacturer and model, but it is usually between 10 and 15 years. After that, loss of accuracy or compatibility issues may occur.

Sometimes, however, replacing the thermostat may be worthwhile even before the end of its service life, especially if your current thermostat is analogue. These thermostats can have a measurement deviation of up to 2 °C. In general, every degree can mean savings of up to 6% in costs, so the investment in a new thermostat pays for itself quickly.

We also recommend purchasing a new thermostat when switching to modern heat sources such as heat pumps or condensing boilers. These heat sources usually require thermostats with OpenTherm communication for modulating output control instead of simple on/off switching. Older thermostat models do not support this communication protocol.

Thermostats we recommend – Honeywell and Siemens

In practice, smart thermostats from leading manufacturers Honeywell and Siemens have proven their worth, and you will also find them in our range.

For undemanding users looking above all for reliability and ease of use, we recommend the Honeywell DT digital thermostat. It is compatible with almost all heat sources and types of heating systems, including older ones. Using the intuitive display, you can easily control both heating and cooling.

If you are looking for a thermostat with remote control capability, the Honeywell Lyric is a good choice. This thermostat allows you to programme a heating schedule for each zone separately, while the settings can be changed at any time via the connected mobile app. An ideal choice for families and busy users who do not want to keep changing settings constantly, but still need to stay in control of their heating even when away from home.

For demanding systems, the Siemens Connected Home intelligent heating control system is suitable, featuring smart thermostats with advanced functions for thermal comfort and energy savings.

Do not wait for the first frosts—check your thermostat now. If it is not working properly, contact us. Our smart heating control specialists will be happy to help you choose a model tailored exactly to your needs.

FAQ – Frequently asked questions about thermostat diagnostics

How can I tell if the thermostat is measuring the temperature incorrectly?

If the room feels colder or warmer than the thermostat display shows, the problem may lie in the temperature sensor or in unsuitable placement of the device. Measurements are often distorted by draughts, direct sunlight, a fireplace or nearby appliances. You can verify its function using an external digital thermometer placed next to the thermostat. A deviation greater than 0.5 °C usually indicates a problem.

Why does the thermostat switch the boiler on and off so often?

Frequent boiler switching is most commonly caused by incorrectly set temperature hysteresis. If it is too low, the thermostat reacts even to small temperature fluctuations and the boiler cycles unnecessarily. The result is higher energy consumption and faster wear of the heating system. The optimum hysteresis is usually between 0.5 and 1 °C.

When is it better to replace a thermostat rather than repair it?

Replacement should be considered especially for thermostats older than 10–15 years or in the case of repeated faults. Older analogue models tend to be less accurate and may increase energy consumption. In addition, new smart thermostats offer more precise control, remote operation and better compatibility with modern heat sources such as heat pumps or condensing boilers. The investment in a new device often pays back within a single heating season.